Before the mid-1970s or so, Porsche quoted dyno figures using the DIN hp standard, which typically come in lower than the SAE standard that is commonly used since then. Porsche switched around that time presumably to take advantage of the higher SAE numbers that all the other manufacturers were already using
DIN hp was measured with all the ancillary stuff still hooked up. In the case of a 911 back in the 70s, that was basically the alternator and fan. SAE hp is measured with the ancillaries being driven by an external source, hence the higher hp number.
I believe the 2.7RS was quoted as 210 DIN hp, and I saw somewhere that the same engine was rated at 230 SAE hp, which seems about right that the fan (and alternator at light duty) consumes roughly 20 hp.
A little reading for those who may be curious:
https://en.wikipedia.org/wiki/Horsepower
Scott